As will be recognized by a person skilled in the art, the phase velocity within a traveling-wave tube (TWT) varies with frequency. As frequency is reduced, the number of helix turns per wavelength increases. As a result, the coupling of electric and magnetic fields between the turns of the helix change, and there is a cancellation of the magnetic flux between the adjacent turns. Consequently, the inductance of the TWT helix decreases, allowing the wave velocity to increase. Additionally, as the frequency is reduced and the number of helix turns per wavelength increases, the electric field created by the helix extends further from the helix.
In order to slow the wave velocity, and consequently the phase velocity, within a TWT, a metal housing is concentrically placed around the helix. Since low frequency signals create electric fields that extend further from the TWT helix, than do high frequency signals, a metal housing can be used to decrease wave velocity at low frequencies while having very little effect on high frequency operation. The effectiveness of the metal housing on slowing wave velocity at low frequencies is proportional to the distance of the metal housing from the TWT helix. Unfortunately, to bring the metal housing closer to the TWT helix also has the accompanying disadvantage of decreasing circuit interaction impedance, which decreases the gain and efficiency of the TWT. Ideally, if a metal housing could be created that conducted only in its axial direction, the effect of the metal housing on circuit impedance could be avoided because no circumferential currents from the TWT helix would flow into surrounding metal shell.
In practice, an axially conducting shell is approximated by a technique commonly referred to as anisotropic loading. In anisotropic loading, a metal shell is concentrically supported around the TWT helix by a plurality of dielectric supports. Shaped conductive members are then attached to the inside diameter of the surrounding metal housing and are extended down toward the TWT helix, in between the dielectric supports. The conductive members are commonly called loading vanes and the dispersion of the TWT helix is controlled by shape and position of the loading vanes relative to both the central helix and the surrounding metal housing. In addition to the presence of the conductive loading vanes, wave velocity within the TWT helix is also effected by the presence of the dielectric supports that separate the TWT helix from the surrounding metal housing. Wave velocity is inversely proportional to the square root of the dielectric constant of the material from which the supports are produced. Consequently, when dielectric material is added in the region surrounding the TWT helix, the wave velocity within the TWT helix decreases.
Referring to FIGS. 1a, 1b and 1c, three typical prior art anisotropic loading configurations are shown for a TWT. In FIG. 1a, a metal housing 10 is concentrically supported around a TWT helix 12 by a plurality of symmetrically disposed dielectric supports 14. On the inner diameter wall of the metal housing, are supported a plurality of conductive loading vanes 16. The dielectric supports 14 and the loading vanes 16 are held in place by being either brazed, adhesively bonded or mechanically fastened to the inner diameter wall of the metal housing. Similarly, the dielectric supports 14 are either brazed, adhesively bonded or mechanically fastened to the TWT helix.
In FIG. 1b, a TWT is shown wherein a plurality of shaped metal clips 18 are used as the loading vanes. The clips 18 also act to hold the dielectric members 14 symmetrically in place around the TWT helix 12. In FIG. 1c a TWT is shown having large solid loading vanes 20. As with the embodiment of FIG. 1a, the solid loading vanes must be either brazed, mechanically fastened or adhesively attached to the metal shell 10.
The embodiments of the prior art do have some substantial disadvantages. In the prior art, the manufacturing of an anisotropic loaded TWT is usually a labor intensive and expensive process. In prior art assembly methods, the insertion of the dielectric supports, and the TWT helix, into the metal shell may be done by heating the metal shell and TWT helix in a vacuum oven. Furthermore, the mass of the loading vanes usually produce an excessive overall shell loading which cause a reduced interaction impedance.
It will be recognized by a person skilled in the art, that such hot insertion manufacturing techniques requires the TWT assembly to cool for hours before it can be tested. Additionally, prior art methods use large conductive elements to form the loading vanes. These conductive elements, either in the form of metal clips or vanes, are expensive to manufacture and require very exacting manufacturing techniques to assemble the TWT helix, metal housing and loading vanes in a concentric orientation. Additionally, the use of separate loading vanes and dielectric supports have made prior art TWT's susceptible to sudden changes in acceleration and other shocks which may dislodge a loading vane element or alter the TWT's concentric construction.
Another disadvantage of many prior art TWTs is the material used to construct the dielectric supports that separates the TWT helix from the metal shell. In prior art TWTs, the dielectric supports are often constructed by beryllia or boron nitride. Both beryllia and boron nitride are expensive materials. Furthermore, the thermal conductivity of both beryllia and boron nitride are limited, creating theoretical limitations on the performance characteristics of a TWT.
In view of the above stated problems in the prior art, it is therefore a primary object of the present invention to provide a anisotropically loaded helix assembly for use in a TWT, and a method for making the same, that produces a TWT that is lower in cost, easier to manufacture, more resistive to shock, has a higher temperature range and operates more efficiently than other common anisotropically loaded TWTs.